There are many forms of "proper roof" If you wan cheap, try this sort
of thing.
http://www.steelroofsheets.co.uk/

Wish I could find something similar in local hardware stores.

Speaking of metal roof, why not just nail a large piece of metal flashing on
top of a plywood and be done? It can even be bent at the edge as a drip
edge. Unfortunately, the local hardware store (HD) only sell roll of
flashing that is about 1 feet wide. What I need is a 5 x 5 feet piece of
flashing.